1. Field of the Invention
The present invention relates to a small, light, and inexpensive imaging optical system for an image sensor.
2. Description of the Related Art
Recently, the use of digital cameras or video cameras including a solid state pickup device such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) has considerably increased. In particular, camera modules offering high megapixel resolution are required in many applications. Manufacturers are always searching for ways to make smaller and lighter digital cameras that can be manufactured at law cost. Thus, an imaging optical system including a single lens is generally used.
Because of the infrared sensitivity of CCD and CMOS devices, an (IR) cut-off filter is generally used between a lens and an image sensor in a filter and lens combination. IR cut-off filters are usually fabricated by coating a piece of flat glass about 30-40 times with an IR cut-off material. Unfortunately, when this kind of IR cut-off filter is combined with a lens, the spectral characteristics of an image formed changes dramatically according to the angle at which light is incident to the lens and filter combination. In other words, light transmittance decreases much faster as the incident angle increases than it would through a lens without a separate IR cut-off filter. IR cut-off filters thus present disadvantages in terms of light transmittance.
Chromatic aberration presents another problem for a conventional lens and IR cut-off filter combination. In many devices, the incident angle of a principal ray can be up to 25° and still be incident on an image sensor. Since there are differences between the transmittances and the spectral characteristics of a center portion of the filter and lens combination and a peripheral portion of the filter and lens combination, large chromatic aberrations are generated. In addition, Moire patterns tend to form on conventional IR cut-off filters, further degrading image quality.
These disadvantages are compounded by the difficulty of manufacturing a lens and IR cut-off filter using lower cost reflow processes. In a reflow process of manufacturing, a camera module is attached onto a printed circuit board (PCB) by melting a solder cream applied to the surface of the PCB. Reflow process manufacturing has been widely used recently because the process of coupling the camera module to the PCB can be simplified, with yield for the process of manufacturing camera modules thereby greatly improved. Demands for developing reflowable camera modules have continuously increased, but the conventional lens and IR cut-off filter combination remains difficult to make in a reflow process.